* Start filling out span backport
* Fix assertion
* Add a span test.
* Fix constructors
* Fill out span tests
* Add test coverage for pointer-count construction
* s/ASSERT_DEATH/EXPECT_DEATH/
* Add bracket operator test
* Add test coverage for other span construction.
* Add test coverage for array construction
* Add test for range construction
* Fix typo.
* Add assignment test
* Add iteration test
* Add data/size functions
* Add array overloads
* Support implicit construction from general containers
* Reformat
* Fix typo
* Fix typo
* Fix enable_if condition